There are plenty of technicolor differences between establishment incumbent Sen. Richard Lugar (R-Ind.) and his more conservative primary opponent, tea party favorite and state treasurer Richard Mourdock, but here’s one thing they do have in common: absurd ads.

A number of the Mourdock campaign spots say this of Lugar, “No wonder they call him Obama’s favorite Republican.” Wait, they do? Sure, thanks to the ads. Prior to that, though, the only “they” was a headline writer for a story that ran on MSNBC’s Web site before Obama was elected, in October of 2008.

Even then, it was posed as a question: “Barack Obama’s favorite Republican? Why the Democrat keeps dropping Dick Lugar’s name.” (Short answer: Because, having worked with Lugar on a bill on cleaning up loose nukes, Obama was eager to cite it as a national security accomplishment, and as proof of his ability to work across the aisle. Though as a McCain campaign spokesman pointed out at the time, “that was a non-controversial issue” opposed by virtually no one in either party.)

Though Obama improbably carried Indiana four years ago, this guilt by association has been in­cred­ibly effective in the state, where the Republican primary is a true tossup.

“When Obama calls him his favorite Republican senator, that’s insulting to me,’’ Jorge Morales, a retired Cummins Engines manager in Columbus, Ind., told me in an interview last month, by way of explaining his decision to back Mourdock in the primary on May 8.

And Lugar, who aired his first negative ad back in February, has gone after Mourdock with ads also based on nothing much. The newest one accuses the state treasurer of using bad judgment in buying up Chrysler junk bonds in ‘08 — investments that did lose the state money when the company went into bankruptcy the next year, though the only state fund that lost the “millions” referred to in the ad was not even administered by Mourdock.

The same ad refers to allegations that Mourdock’s campaign manager encouraged a raid on a state GOP voter database. (And did he? Well, the unsubtle e-mail in question, from campaign manager Jim Holden, did encourage those with access to “start pillaging e-mail addresses like a Viking raider attacking a monastery full of unarmed monks.”)

The allegation that most seemed to rile Mourdock — his temper flared at the mention of it during an interview in April — is that he unfairly got a homestead exemption on both of his two homes, when Hoosiers can legally only claim the tax break on their primary residence. The charge is true, but county officials have said it was their error, not Mourdock’s.

(You can’t help thinking that if Lugar, who’s been busted for selling his Indiana house after he was first elected to the U.S. Senate in 1977, had had just one of his rival’s two homes, they’d have had a lot less to argue about.)

Lugar has also charged that if elected, Mourdock would be beholden to outside interests. In that one, there’s a clip of Mourdock saying, “I’m confident there will be a lot of outside money flowing in to help us,” as indeed there has, from the National Rifle Association, FreedomWorks, and the Club for Growth.

And since February, Lugar has been charging that Mourdock misses a lot of the meetings for the boards he sits on — again, true, but he sits on so many of these that if he attended them all he’d do nothing else, and he usually sends a designated representative.

Finally, last week, the Lugar campaign made a big deal of digging up an old candidate questionnaire that showed that 20 years ago, Mourdock said he favored the Fairness Doctrine guaranteeing equal air time for all points of view, and what’s more, supported a government-funded free year of college to all who graduated from high school with at least a “B” average. Longtime Lugar aide Andy Fisher called this proof that Mourdock “shifts his position on core conservative values.’’ Maybe what he meant to say is that Mourdock had better sense back then?

By any measure, this is one negative race, but its brutality is being seriously exaggerated, too. The race is “scathing,’’ CNN says, and “getting more vicious by the day,’’ the conservative site Newsmax reports.

“Dick Lugar is under vicious attack in a Primary Election precisely because he consistently puts country first -- including, first before party orthodoxy,’’ a fellow Rhodes scholar wrote in a recent fund-raising letter.

Maybe, but when Mourdock’s eyes filled with tears as he told me about the low-down charges his wife even had people asking her about at church, I was surprised that the despicably low blow in question involved a duplicate homestead exemption.

At the time, Hillary Clinton’s “3 a.m. phone call’’ ad against Obama in the ‘08 primary was criticized as a low blow. And the cumulative effect of the $65 million spent on ‘10 ads in which Nancy Pelosi was variously portrayed as a dog, a monster, and a witch made the cut in my mind.

I grew up all of 2.5 miles from the Indiana line, so can’t claim to fully understand the Hoosier mind, though I did go to college in the state, and my family lives there now.

But gosh, Indiana, to hit “scathing,” you’re going to have to do more than mine meaningless attendance records or plump the pillows of imagined presidential admiration. And God willing, you won’t even try.

Melinda Henneberger is a Post political writer and anchors the paper’s ‘She the People’ blog. Follow her on Twitter at @MelindaDC.

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